CMOS based image sensors have become an active area of development because they, unlike CCD sensors, are made with process steps that are compatible with those used to make standard CMOS digital integrated circuits. An expectation has evolved that ultimately, CMOS based image sensors will be more cost effective than CCD based image sensors because they can be produced on one integrated circuit using economical CMOS based processes.
There are several characteristics of importance for image sensors. Among these are power consumption, fill factor, dynamic range, noise, and uniformity. Fill factor is the percentage of the resulting imaging device that is used for detecting light.
Several investigations of CMOS based image sensors have been conducted to improve some of these characteristics for monochrome or full color imaging devices. One of these is described in U.S. Pat. No. 5,461,425 to Fowler et al., in which the analog signal from each pixel in an array area is converted by digital circuitry at each pixel to serial bit streams. These serial data streams are analyzed by a central processing circuitry of the array by counting the bits of one state to obtain an approximation of the original analog value. This improvement helps noise and dynamic range by converting the small charges from the photodiode junction to digital information at each pixel. Another investigation is described in an article published in a record of the IEEE Custom Integrated Circuits Conference of 1996 entitled “A 128×128 Pixel CMOS Area Image Sensor with Multiplexed Pixel Level A/D Conversion” by Yang et al. that describes multiplexing monochrome pixels in groups of 4 to improve noise and dynamic range.
Investigations into those aspects of image sensor design that are unique to color imaging are also ongoing. One of these is described in U.S. Pat. No. 5,965,875 to Merrill, in which a triple-well structure is described that co-locates the sensors for three color bands (red, green, blue) essentially on top of each other. This directly affects the fill factor by reducing the area needed to capture light in different color bands compared to a conventional side by side arrangement for color imagers.
What is still needed is a comprehensive cost effective solution for a color imaging integrated circuit.
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